It has been a long time in coming, but the University of California has updated the guidelines for nutrient sufficiency in strawberry plants. The last such publication was released in 1980, over 30 years ago. The study that led to the latest guidelines was funded by the California Strawberry Commission and was enabled by the cooperation of numerous strawberry growers in the Santa Maria and Watsonville-Salinas regions.
The results from the study for Nitrogen, Phosphorus, and Potassium levels is summarized as follows:
Petiole NO3-N was so highly variable as to be nearly worthless as a diagnostic technique; during peak fruit harvest (our sampling dates 3 and 4) petiole NO3-N in high yield fields varied from < 200 PPM to 2,600 PPM. While we believe that leaf total N is a more reliable measurement, this study suggests that maintaining petiole NO3-N > 1,000 PPM pre-harvest, and > 400 PPM during peak harvest, is adequate to maintain high productivity. Given the high variability of petiole NO3-N it is possible that concentrations < 400 PPM would be adequate during the summer.
Petiole PO4-P and K were less variable than petiole NO3-N. Maintaining PO4-P > 1,200 PPM throughout the season should ensure P sufficiency. Given the high soil P availability in most coastal soils rotated with vegetable crops, this level is probably much higher than the ‘critical value’. Maintaining petiole K > 2.5% preharvest, and > 1.5% during peak harvest, appears to be adequate.
Additionally, Calcium, Manganese, Iron, Nitrogen, Zinc, and Copper were more variable and less crucial than N, P, and K levels. For the full report, see here: Strawberry Plant Nutrient Sufficiency Levels Revised.